The present invention relates to methods of preparing diaphragms consisting essentially of asbestos that are useful in electrolytic cells utilized for the electrolysis of aqueous salt solutions, especially useful in electrolytic cells utilized for the electrolysis of aqueous alkali metal halide solutions, e.g., sodium chloride brine.
Diaphragms are used in electrochemical processes to separate an anolyte liquor from a catholyte liquor while permitting the flow of electrolyte therethrough. Diaphragms are used, for example, to separate an oxidizing electrolyte from a reducing electrolyte, a concentrated electrolyte from a dilute electrolyte, or a basic electrolyte from an acidic electrolyte.
In the electrolysis of an aqueous alkali metal halide solution, the diaphragm separates an acidic anolyte from an alkaline catholyte. Historically, commercial chlor-alkali diaphragms have been made of asbestos. Such diaphragms have been prepared by vacuum-drawing a liquid slurry containing asbestos fibers onto a porous cathode thereby depositing a mat of asbestos on the cathode. Asbestos diaphragms typically are characterized by a short lifetime of about 6 to 8 months.
Numerous efforts have been made to improve the lifetimes and performances of asbestos diaphragms. For example, asbestos diaphragms, vacuum-deposited from an aqueous slurry containing sodium hydroxide, have been strengthened by heating the diaphragms to a temperature between about 110.degree. Centigrade (C) and 280.degree. C. for a sufficient period of time to react the sodium hydroxide and the asbestos, as taught by U.S. Pat. No. 3,991,251. However, these diaphragms can be too impermeable to the flow of electrolyte therethrough, thereby requiring a higher hydrostatic head of brine on the anolyte side of the diaphragm to maintain the desired electrolyte flow.
The following patents illustrate the use of materials, in particular fluorine-containing polymers, as binders with asbestos diaphragms. Generally, this technique includes mixing a slurry containing a particulate or fibrous binder material and asbestos fibers, depositing the solid materials as a mat on a porous cathode, and heating the diaphragm mat to sinter or melt the binder material, thereby effect bonding between the asbestos and binder. For example, U.S. Pat. No. 4,065,534 describes the codeposition of a thermoplastic resin with asbestos followed by melting the resin to bind the asbestos. U.S. Pat. No. 4,070,257 describes the preparation of a diaphragm mat containing asbestos and a fluorine-containing resin binder, followed by sintering the resin by heating the diaphragm for about 1 to 10 minutes at a temperature from the melting point of the resin to 100.degree. C. above the melting point of the resin. U.S. Pat. No. 4,142,951 describes depositing a diaphragm of crocidolite asbestos, chrysotile asbestos and a polymeric fluorocarbon resin followed by heating to sinter the polymer resin and provide bonding between the asbestos and resin. Finally, U.S. Pat. No. 4,410,411 describes the codeposition of a fluorine-containing polymer and asbestos followed by heating at a temperature sufficient to fuse, soften, and flow the polymer and thereby bind the diaphragm.